This invention relates to an anti-afterburn system for an engine of a vehicle in which secondary air is supplied to an exhaust manifold of the engine for purifying exhaust gas.
In order to purify the exhaust gas, it has been known that secondary air is supplied to thermal reactors or catalytic convertors to reburn positively unburned gas to be discharged from the engine before it is discharged into the atmosphere. In this case, if the exhaust gas containing a large amount of unburned gas is discharged from the engine to the exhaust manifold, it reacts with the secondary air within the reactor or convertor, thereby causing an explosion or afterburn. When a throttle valve of a carburetor is opened or closed rapidly, richer air-fuel mixture is supplied to the engine and not burned completely in a combustion chamber thereof, so that said gas containing the unburned gas is discharged into said exhaust manifold to increase the generation of afterburn.
In conventional technics relating to anti-afterburn, there has been provided devices in which the supply of secondary air is terminated or fresh air is supplied to an intake manifold upon deceleration. However, in these devices, delay of operation thereof is occured and due to this delay the afterburn is generated by the secondary air supplied before the deceleration.
Also, in an engine having a secondary air supply means, an exhaust gas purifying device provided in an exhaust system of the engine is heated by combustion heat of unburned gas contained in the exhaust gas. Particularly, in high loading running condition of the engine, said exhaust gas purifying device will be damaged by the heat and cause a fire.